I think I need to put my own thinking cap back on John, ignore what I said about the thermoswitch earth cable...
Run a positive cable from an ignition fused live to the switch to a new relay in the engine bay, through the relay to earthto operate the solenoid in the relay. Run another cable from the power side of the thermoswitch through the controlled circuit of the relay to earth.
This circuit will stop the fans running when the ignition is off unless the thermoswitch knows the water temp is above it's operating limit. Yes, when you come back to the car the fans will still run but you should hear them running and can switch off if you need to.
If the fans switching on on restart bothers you there is another solution.
https://www.carbuilder.com/uk/rear-fog-light-controller-for-new-iva-rules-for-momentary-switchIf you wire one of these rear foglight controllers into the circuit it will prevent the fans running on restart until you turn the switch off and on again. It's designed to stop you running with your foglight on if you forgot to switch it off when you last used/needed it.
Or you could wire in a warning light or an alarm buzzer?
Guy